kne image plus persistence means that the iso size is too large I believe

The check in /home/src/wubi/trunk/src/wubi/frontends/win32/cd_menu_page.py (line 63) is too restrictive, that was in place to discriminate against DVD. A quick fix is to remove the upper bound

if self.info.force_wubi or \
        self.info.cd_distro.min_iso_size < used:

DVDs will still be discriminated when the user is online since the MD5 check will fail, but DVDs will go through when offline and in this case the free space check is inappropriate as we check for regular ISO size. In short removing the upper bound is an issue if the user is using a DVD, offline, and with low disk space.

A proper DVD check would be a bit too invasive at this stage.

As a quick and dirty patch we could have: min_iso_size < used < DVD_size
Still inadequate but will provide some headroom without side effects.

Documented at <https://wiki.ubuntu.com/KarmicKoala/ReleaseNotes#Wubi%20using%20persistent%20storage%20on%20a%20USB%20disk>:

Users who wish to run Wubi from a USB disk that has persistent storage enabled will need to run it with the {{{--force-wubi}}} option from the Windows command line. (Bug:461566)

This is still a problem. When you install Wubi from a USB, it copies the entire USB partition onto the hard drive (can easily be 8GB or 16GB these days). Then Wubi fails the ISO if it's larger than 900000000 bytes (~852MB). When you create an Ubuntu USB it states to have at least 2GB free space: http://www.ubuntu.com/download/ubuntu/download "Insert a USB stick with at least 2GB of free space".

Since the precise desktop ISO is supposed to be 750MB I suspect more people will be using USBs.

There are two major problems:
1. The install fails. obviously.
2. You could potentially full up someone's hard drive since there's no space check to prevent a 16GB ISO being copied.